The Study And Comparison Of Colossal Magnetoresistance In La_0.7-xEu_xCa_0.3MnO₃ And La_0.7Ca_0.3MnO₃/(Eu₂O₃)_x

Recently, there are a lot of experimental and theoretical works focused on manganites. This is mainly because these materials can exhibit extraordinary colossal magnetoresistance in the presence of magnetic field and its potential application in industry. Moreover, manganites are typical strongly correlated electron systems, spin, charge, and orbital degrees of freedom are coupling simultaneously, and the classical simplifications that neglect some interactions to study the properties of manganites do not work. Therefore, to understand the mechanism of colossal magnetoresistance in manganites is a main challenge in condensed matter physics. Though Zener's famous double exchange theory can explain the ferromagnetism/metal --paramagnetism/insulator transition and colossal magnetoresistance successfully, but it is only qualitatively not quantitatively due to neglecting the existence of polaron formed by lattice distortion in this theory. So the study of polaron is of great importance to understand manganites.In this thesis,we fabricate the La_0.7-xEu_xCa_0.3MnO₃, x=0.01, 0.03, 0.10, 0.15 by conventional solid state reaction, and La³⁺ is partially substituted by Eu3+. The experimental results indicate that with the increase of Eu³⁺ dopant, the average ion radius of A-site decease, and size disorder increase. In order to study the high temperature (T>TP) conduction mechanism, we have used four theoretical models to fit the experimental data. We found that with the increase of dopant, the single model can not fit the data very well, so we propose that there are spin polaron and lattice polaron coexistence in these material. Meanwhile, the ferromagnetism/metal-paramagnetism/insulator transition temperature decrease, and the magnitude of colossal magnetoresistance is increased.Besides, we also fabricate composite La_0.7Ca_0.3MnO₃/(Eu₂O₃)x, x=2%, 3%, the sintering temperature is 900℃,1100℃and 1300℃. The grain boundary effect is very large as for TS =900℃and 1100℃, the samples exhibit a large magnetoresistance far blow curie temperature TC. With the increase of sintering temperature, the noninstrinsic magnetoresistance caused by grain boundaries effect weakened.At the end of this thesis, we compare the magnetoresistance in doped manganites with composite.